How many young lives would you guess have been claimed in gang-related warfare in Los Angeles over the past four decades? 100? 200? 500? Try 15,000 and counting. What makes a poor kid pick up a gun and shoot another poor kid for something as seemingly meaningless as a pair of sneakers or for passing through his neighborhood?

To get at the roots of such profound dysfunction, you really have dig rather deeply, as does director Stacy Peralta in "Crips and Bloods: Made in America." Narrated by Forest Whitaker, this heartrending expose' is set to debut this week on the PBS-TV series Independent Lens. (Check local listings.)

The program opens with actual footage of gang-bangers being blown away in drive-bys and being left lying dead in the street. Apparently, some of these demented killers are so proud of their slayings that they get a kick out of filming their dirty work so they can watch it later at their leisure. Thus, it is strongly suggested that Crips and Bloods be taken on a full stomach.

Still, such sensational and sobering moments aside, the picture more importantly offers a serious discussion of exactly how the gangs in L.A. came to be. It makes no bones about indicting a segregated city where disadvantaged Black boys were discouraged from joining Cub Scout or Boy Scout troops located in lily-White communities, leaving the generally-fatherless African American adolescents to fend for themselves in the ghetto.

The folks from South Central interviewed here make it abundantly clear that the LAPD perceived all Black males as criminals and thus saw it as their duty to keep them from ever crossing the invisible borders into White enclaves. Consequently, it was not unusual for a kid from the 'hood to go from the cradle to the grave without ever seeing the suburbs, the ocean at Malibu, the mansions in Beverly Hills, or other alternatives to the thug life.

According to former gang member Ron Wilkins, the Crips and the Bloods were originally formed as benign, street-front fraternities which offered rudderless youngsters a sense of status, family, power and acceptance in a world which was showing them little in the way of love. But they gradually morphed into felonious associations, since there weren't many legal outlets for all that unbridled testosterone.

The crack epidemic of the 1980s didn't help matters much, nor did the dwindling manufacturing base or an educational system way too willing to graduate functional illiterates. No wonder inhabitants of the 'hood exhibit the same level of post traumatic stress syndrome as people living in a war zone.

Before the curtain comes down on this daunting documentary, expect to well up while watching emotional tableaus of grieving mothers burying their babies at funerals and simply staring blankly into the camera with tears streaming down their pained faces. That's the tragic side of the thug lifestyle that they never show in gangsta videos.